1. Field of the Invention
The present invention relates to a field effect transistor that uses gallium nitride-based compound semiconductor.
2. Description of the Related Art
Since a gallium nitride-based compound semiconductor is wide in the forbidden band, a field effect transistor (FET) using that semiconductor is capable of being operated at a high frequency and at a high withstand voltage. So, it is expected as a high-output power semiconductor device. And there have up to now been proposed a metal semiconductor FET (MESFET), a high electron mobility transistor (HEMT), and so on. The HEMT is a high-speed semiconductor device which has become popular as the devices operating at a high frequency. Specifically, regarding this, a device that uses a GaAs/AlGaAs heterojunction has been put to practical use. Because of its excellent microwave/millimeter wave characteristics, that device has widely been used as a low-noise high-speed field effect transistor for, for example, satellite broadcasting receiver (refer to, for example, Japanese Patent Application Laid-Open No. 2003-297856).
In the recent years, attention has been drawn toward the HEMT which uses a GaN compound (hereinafter referred to as the GaN-based HEMT) in place of a GaAs compound as the next generation high-speed type FET. The GaN compound is wide in band gap and also the saturation electron mobility, which is estimated from the effective mass of electron, is high. Thus, the possibility exists of realizing a high frequency device that provides a high output and that can operate at a high withstand voltage and at a high temperature. Because of this, extensive studies of this HEMT and researches have been made. An example of the HEMT structure which uses a GaN compound is illustrated in FIG. 1. The GaN-based HEMT illustrated in this figure includes an insulating sapphire substrate 11, on which there are sequentially laminated an AlN buffer layer 12, an undoped GaN layer 13 acting as an electron transit layer, and an n-type AlGaN layer 14 acting as a carrier supply layer. Also, on an upper surface of the n-type AlGaN layer 14 there are formed a source electrode 15, a gate electrode 16, and a drain electrode 17. In this structure of HEMT, the n-type AlGaN layer 14 acting as the carrier supply layer supplies electrons to the undoped GaN layer 13 acting as the electron transit layer, and the electrons that have been supplied transit, with a high mobility, at an uppermost portion of the GaN layer 13, using as a channel a region 13a contacting with the n-type AlGaN layer 14.
In order to increase the output of the above-described HEMT, there is the need to make it high in withstand voltage so that it can be applied with a high voltage. However, when a high magnitude of voltage is applied between the source electrode 15 and the drain electrode 17, the speed of the electron E transiting at or through the channel region 13a increases to collide with the relevant lattices. Due to these collisions, collision ionization occurs creating new collision-ionized electrons E′ and holes h. The produced carriers, one after another, are repeatedly collision ionized, resulting in that normal FET operations become unable to occur. The problem regarding the collision ionization is that the holes that have been produced are stored in the electron transit layer. That is, of the ionization electrons and holes that have been produced, the electrons are used as electric current and have no problems with storage. However, since the holes are not contributed as electric current, positive electric charges are stored at a lower part of the channel. The positive electric charges that have been stored cause the induction of negative electric charges toward them and, by doing so, draw a larger number of electrons toward them. As a result of this, collision ionization is promoted, which results in a larger number of holes being stored and causing a rapid increase in the electric current. This goes beyond the limit of the withstand voltage of the device to cause breakdown of it.
For the purpose of preventing the above-described drawback, a structure has been proposed which is disclosed in Japanese Patent Application Laid-Open No. 2001-168111. This structure, as illustrated in FIG. 2A, is made into a construction wherein a p-type GaN layer 28 is formed under a channel 23 of an undoped GaN layer, and, by doing so, the channel 23 is interposed between the p-type GaN layer 28 and the undoped AlGaN layer 24 and a gate electrode 26. As a result of this, the holes that have been produced due to the collision ionization when having passed an electric current between a source electrode 25 and a drain electrode 27 become able to be drawn out of the device via the electrode 29 formed on the p-type GaN layer 28. In this way, it is possible to increase the withstand voltage.
However, to realize this construction, there exist very difficult problems to solve. In order to manufacture a GaN-based HEMT of the structure of FIG. 2A, as illustrated in FIG. 2B, it is necessary to form the p-type GaN layer 28 on a substrate 21 through a buffer layer 22. An example of known methods to make the GaN layer p-type is annealing treatment in which the GaN layer doped with p-type impurity such as Mg is heated to remove hydrogen. However, it is not easy to make hydrogen removal, in a structure wherein on the Mg doped GaN layer grown on a sapphire substrate there are further laminated layers, to such an extent, by annealing treatment, as the Mg-doped GaN layer which is located under exhibits a p-type one. In addition, since forming the p-type GaN layer results in forming a large capacitance over an entire surface of p-type layer, there is also the problem that it becomes difficult to make the device operation higher in speed. Further, since, regarding the p-type layer, there is the need to dope a large amount of impurity, the crystallinity of the p-type layer tends to be very bad. This is a problem that is peculiar to a gallium nitride-based compound semiconductor. Accordingly, when providing a p-type layer on the substrate side of the channel, the crystallinity per se of respective layers formed on the p-type layer (whose crystallinity is inferior) becomes deteriorated. This results in the characteristics of the device being deteriorated.
Also, when applying a voltage to the HEMT of the electrode structure such as that illustrated in FIG. 1, an electric field which is higher than the electric field applied in the other part is inconveniently applied at the drain terminal of the gate electrode. When a high voltage is applied to the field effect transistor device and as a result a local field intensity exceeds a specified value, the above-described breakdown of the device takes place at that place. So, to increase the withstand voltage of the device, it is important to avoid the localized electric field.